Conventionally, transparent plastic molded objects (hereinafter “transparent object(s)”) are used in various fields such as food wrappers, household electrical appliances, vehicle parts and the like and also widely used as optical parts such as displays, optical disks and the like. However, when such a transparent object is treated as an object to be imaged, it may be difficult to properly detect the transparent object with a typical camera that can detect only luminance information. This is because the transparent object has a low reflection rate and high transmissivity. Therefore, an obtained contrast between the transparent object and the background is likely to become unclear.
To resolve the problem, Patent Document 1 discloses a transparent object detection method to be used in a transparent body detection system of FIG. 1. As schematically illustrated in FIG. 1, the transparent body detection system includes a light projecting side polarization plate 95, a light projecting device 91, a polarization apparatus 92 having a light receiving side polarization plate 96, a TV camera 93, and an image processing apparatus 94. The light projecting side polarization plate 95 polarizes light in a specific direction. The light projecting device 91 projects light to a predetermined region including a transparent object 90. The light receiving side polarization plate 96 transmits only specific polarized light from the light having transmitted through the predetermined region. The TV camera 93 images a two-dimensional image based on an image obtained by receiving light having transmitted through the light receiving side polarization plate 96. The image processing apparatus 94 detects the transparent object 90 based on projected light distribution of the image obtained by the TV camera 93. By having this configuration, the transparent object 90 can be detected. To that end, light polarized in a specific direction is projected to a predetermined region including the transparent object 90. Then, the light having transmitted through the predetermined region is further transmitted through the light receiving side polarization plate 96 that transmits only a specific polarized light. The light transmitted through the light receiving side polarization plate 96 is received by a CCD (Charge Coupled Device) of the TV camera 93. Based on the projected light distribution in an image obtained by the CCD, the transparent object 90 is detected.
However, according to the technique described in Patent Document 1, the transparent object is detected using birefringence (=double refraction) characteristics of the transparent object. Because of this feature, it may be difficult to detect a transparent flat plate having no birefringence characteristics such as a glass flat plate when the technique described in Patent Document 1 is applied.
On the other hand, as a technique of detecting an image of a plate-like object, Patent Document 2 discloses a surface inspection apparatus as illustrated in FIG. 2. As schematically illustrated in FIG. 2, the surface inspection apparatus forms an image indicating intensity distribution of polarized light by detecting polarized lights corresponding to different reflected lights from a surface of a steel plate 81 using line sensor cameras 82a through 82c.     [Patent Document 1] Japanese Laid-Open Patent Application No. 2002-98650    [Patent Document 2] Japanese Laid-Open Patent Application No. 09-166552